720963 ÷ 21 
The aim is to understand the frequency of 21 in 720963.

Going ahead to step 1:
If we divide 7 by 21, we get 0 and a remainder of 7.
The next digit of our result is 0. 
Result so far: 0.0
If we subtract 0 from 7, we get 7.
Append the next digit (2) from the dividend to 7 and continue with: 72 / 21

Advancing to step 2:
21 goes into 72 3 times with a remainder of 9.
Use 3 as the next digit of our solution. 
Result so far: 3.0
The remainder is 9 after subtracting 63 from 72.
Fetch the next digit (0) from the dividend, attach it to 9 and continue: 90 / 21

Moving on to step 3:
When dividing 90 by 21, we get 4 with a remainder of 6.
Write down 4 as next digit of the result. 
Result so far: 34.0
Subtract 84 from 90 to get 6.
Fetch the next digit (9) from the dividend, attach it to 6 and continue: 69 / 21

Going ahead to step 4:
When dividing 69 by 21, we get 3 with a remainder of 6.
The next digit of our result is 3. 
Result so far: 343.0
Subtract 63 from 69 to get 6.
Grab the next digit (6) from the dividend, add it to 6, then carry on: 66 / 21

Advancing to step 5:
66 divided by 21 is 3 with a remainder of 3.
The number 3 becomes the next digit in our result. 
Result so far: 3433.0
Subtract 63 from 66 to get 3.
Include the next digit (3) from the dividend after 3, then repeat: 33 / 21

Let's proceed to step 6:
21 can be fit into 33 1 times, resulting in a remainder of 12.
Use 1 as the next digit of our solution. 
Result so far: 34331.0
The remainder is 12 after subtracting 21 from 33.
Since there are no more digits in the dividend, we add a zero to the remainder making it 120, and continue the process.

Moving on to step 7:
21 goes into 120 5 times with a remainder of 15.
Write down 5 as next digit of the result. 
Result so far: 34331.5
Subtract 105 from 120 to get 15.
Since there are no more digits in the dividend, we add a zero to the remainder making it 150, and continue the process.

Step 8:
The number 21 fits into 150 7 times, leaving a remainder of 3.
Put 7 as the next digit of the answer. 
Result so far: 34331.57
If we subtract 147 from 150, we get 3.
Since there are no more digits in the dividend, we add a zero to the remainder making it 30, and continue the process.

Moving on to step 9:
When dividing 30 by 21, we get 1 with a remainder of 9.
Record the quotient 1 as the next digit in the result. 
Result so far: 34331.571
The remainder is 9 after subtracting 21 from 30.
Since there are no more digits in the dividend, we add a zero to the remainder making it 90, and continue the process.

Step 10:
90 divided by 21 is 4 with a remainder of 6.
Put 4 as the next digit of the answer. 
Result so far: 34331.5714
Deduct 84 from 90 and we're left with 6.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

On to step 11:
The number 21 fits into 60 2 times, leaving a remainder of 18.
The next digit of our result is 2. 
Result so far: 34331.57142
The remainder is 18 after subtracting 42 from 60.
Since there are no more digits in the dividend, we add a zero to the remainder making it 180, and continue the process.

Going ahead to step 12:
When dividing 180 by 21, we get 8 with a remainder of 12.
The next digit of our result is 8. 
Result so far: 34331.571428
If we take 168 away from 180, we end up with 12.
Since there are no more digits in the dividend, we add a zero to the remainder making it 120, and continue the process.

Advancing to step 13:
The number 21 fits into 120 5 times, leaving a remainder of 15.
The next digit of our result is 5. 
Result so far: 34331.5714285
Subtracting 105 from 120 leaves us with 15.
Since there are no more digits in the dividend, we add a zero to the remainder making it 150, and continue the process.

Let's proceed to step 14:
21 can be fit into 150 7 times, resulting in a remainder of 3.
Use 7 as the next digit of our solution. 
Result so far: 34331.57142857
Subtracting 147 from 150 leaves us with 3.
Since there are no more digits in the dividend, we add a zero to the remainder making it 30, and continue the process.

The final outcome is 34331.57142857, with a leftover of 3e-09.